It is well known that kaolin can be formed into particles, alone or with other ingredients such as particles of zeolitic molecular sieve, to form coherent bodies such as microspheres which, when calcined, are further hardened. For example, essentially catalytically inert microspheres adapted to be used in a selective vaporization process or to be blended with active zeolite particles are produced by spray drying a slurry of hydrous (uncalcined) kaolin arid calcining the resulting microspheres. See U.S. Pat. No. 4,781,818, Reagan, et al. Microspheres consisting of calcined kaolin and impregnated with precious metal have been commercially used to promote CO combustion in fluid catalytic cracking units. See U.S. Pat. No. 4,171,286, Dight, et al. In some cases, the promoter particles are preblended with particles containing an active cracking catalyst component (usually zeolite Y). In other applications, the promoter particles are introduced at a suitable level into the regenerator of an FCC unit, separately from the particles of cracking catalyst. Still another use of microspheres composed of calcined kaolin is as a reactant with caustic or sodium silicate solution to form zeolitic cracking catalyst by so-called in-situ routes. See, for example, U.S. Pat. No. 4,493,902, Brown, et al. Many cracking catalysts are prepared by mixing a slurry of previously formed crystals of zeolite Y in appropriate ion-exchange form with silica sol or silica alumina sol and kaolin followed by spray drying. Spray dried microspheres of calcined clay may also be used as a fluidization additive in FCC units.
In carrying out various processes in which an aqueous slurry of kaolin is spray dried, it is conventional to disperse the kaolin in the slurry prior to spray drying in order to permit the formation of high solids slurries that are sufficiently fluid to be spray dried. High solids are preferred for economic reasons. Also, higher solids are conducive to the formation of more strongly bonded particles. To disperse kaolin in water, conventional anionic clay dispersants such as sodium condensed phosphate salts, sodium silicates, soda ash, sodium polyacrylate and mixtures thereof are used. Typically, the pH of concentrated dispersed slurries of kaolin are mildly alkaline to neutral, e.g., 6.0 to 8.0, with pH 7 being optimum.
In many catalytic processes, such as FCC processes, the particles must be attrition-resistant as well as sufficiently porous. Generally, one of these qualities is achieved at the expense of the other. For example, as a particle of given chemical composition is formulated to be highly porous, the hardness usually decreases.
U.S. Pat. No. 5,190,902, Demmel, utilizes the addition of phosphoric acid (or other phosphate compounds) with kaolin in a spray drying process to produce spray dried microspheres which are then calcined. In some formulations zeolite particles are present in the spray dryer feed. The process is carried out in one of two basic ways. In one, the slurry of clay particles is brought to a low pH, e.g., 1.0 to 3.0 before being mixed with a source of phosphorus, followed by spray drying. In the other, the clay slurry is brought to a high pH level (e.g., 14.0 to 10.0) before mixing with phosphate-containing compound. According to the teachings of this patent, use of these pH ranges is necessary for the production of particles with superior attrition resistance. A significant problem with these prior art approaches to producing calcined clay microspheres is that neither pH range is the mildly alkaline to neutral pH range at which concentrated slurries of kaolin are fluid and amenable to commercial spray drying using high solids slurries. Thus, the patentee diluted the original 70% solids slurry to 40% before pH adjustment apparently because of viscosity increases which follow formation of the aluminum phosphate binder.
Similarly, U.S. Pat. No. 5,231,064, and U.S. Pat. No. 5,348,643, both Absil, et al, describe formation of a cracking catalyst by spray drying a slurry of zeolite with a slurry of clay treated with a phosphorus source at a pH less than 3. Sufficient water is added to bring the combined slurries to a low solids content of ca. 25%.
The use of aluminum phosphates as a binder and hardening agent is well known in the ceramics industry (F. J. Gonzalez and J. W. Halloran, Ceram. Bull 59(7), 727 (1980)). This usually involves addition of alumina to the ceramic mix, followed by treatment with phosphoric acid, curing and firing. Similarly, the hardening of aluminous masses such as those composed of bauxite or kaolin by incorporation of phosphoric acid followed by heat treatment also is known. The product of this treatment is apparently an aluminum phosphate which can act as a binder. An aluminum phosphate formed by interaction of phosphoric acid solution with an aluminum salt solution has been used to bind zeolite and clay in a cracking catalyst composition (U.S. Pat. No. 5,194,412).
Commonly assigned U.S. Pat. No. 5,521,133 discloses forming improved porous microspheres based on spray dried calcined kaolin. The phosphoric acid and kaolin are pumped in separate streams to a static mixer that is adjacent to the atomizer of a spray dryer. The phosphoric acid is injected into a dispersed high solids kaolin slurry and the slurry is virtually instantaneously atomized into droplets in a spray dryer. The term “virtually instantaneously” as used therein refers to a time less than about 20 seconds, preferably less than about 10 seconds. This spray drying technique eliminates undesirable kaolin flocculation and agglomeration prior to the spray dryer.
Kaolin flocculation and agglomeration prior to the spray dryer would result in relatively large clay particle aggregates in the spray dryer feed. The presence of these large aggregates cause poor and uneven packing of the kaolin particles in the microspheres resulting from the spray drying process. Poor and uneven packing of kaolin particles in microspheres leads to insufficient interparticle binding of the particles within the microspheres. This results in poor physical properties including poor attrition resistance.
In contrast, the process of U.S. Pat. No. 5,521,133 provides microspheres which have good kaolin interparticle binding and excellent physical and chemical properties. For example, microspheres produced by the patented process have high attrition resistance. In addition, the microspheres retain higher porosity than microspheres from the same kaolin that are spray dried without phosphoric acid binder and are calcined to the same temperature. This porosity increase coupled with higher attrition resistance is surprising since, generally, an increase in porosity leads to a decrease in attrition resistance. Sufficient porosity is also important because the physical properties of the microspheres should be comparable to those of microspheres containing the active zeolitic catalytic component, i.e., very low or very high densities are undesirable.
Microspheres prepared by using principles of U.S. Pat. No. 5,521,133 have several applications in FCC including: catalytically inert microspheres having a high attrition resistance; active cracking component (by adding zeolite to the clay slurry); microspheres (with or without added components such as MgO) that preferentially react with contaminant vanadium; microspheres for in-situ zeolite growth (see, for example, U.S. Pat. No. 4,493,902, Brown et al); fluidization additive and catalytic support for a carbon monoxide combustion additive.
An FCC additive containing kaolin and 10-25% by weight ZSM-5 has been used to improve gasoline octane and to enhance LPG yields. To further increase LPG while minimizing unit activity loss due to dilution, additives with ZSM-5 levels greater than 25% are required. Unfortunately, in microsphere additives that contain higher than 25% ZSM-5 levels the attrition resistance of the microspheres becomes an issue. An objective of this invention is to make an FCC additive containing at least 40% by weight ZSM-5 with the attrition resistance and with an activity similar or better, compared on the per unit ZSM-5, than the additive containing 25% ZSM-5 or less.
W. R. Grace has a U.S. published patent application, U.S. 2003/0047487, for making additives containing 40-80% ZSM-5 with good attrition resistance. As described therein, the amount of added alumina in the microsphere formulation needs to be less than 10% and the total alumina (added alumina plus alumina in clay and zeolite) is less than 30%. The added alumina has a BET surface area greater than 50 m2/g, preferably greater than 140 m2/g. In their process, clay, zeolite, alumina, and binders such as phosphoric acid and aluminum chlorohydrol are all mixed together to form a uniform slurry prior to spray drying.
The present inventors have discovered that when at least a portion of the microsphere contains particles of high density, low surface area unreactive species, microspheres containing 30% or more ZSM-5 possess high activity per unit ZSM-5 and superior attrition resistance compared to microspheres without such low surface area unreactive species.